U.S. patent application number 11/124053 was filed with the patent office on 2005-09-29 for method and apparatus for producing semiconductor device.
This patent application is currently assigned to Semiconductor Energy Laboratory Co. Ltd., a Japan corporation. Invention is credited to Tanaka, Koichiro, Yamazaki, Shunpei.
Application Number | 20050214991 11/124053 |
Document ID | / |
Family ID | 34624004 |
Filed Date | 2005-09-29 |
United States Patent
Application |
20050214991 |
Kind Code |
A1 |
Yamazaki, Shunpei ; et
al. |
September 29, 2005 |
Method and apparatus for producing semiconductor device
Abstract
An amorphous silicon film is formed on a flat glass substrate,
and then crystallized by heating to obtain a crystalline silicon
film. The glass substrate is placed on a stage having a convex
U-shaped curved surface. The glass substrate is heated for a
desired period of time at a temperature close to a strain point of
the glass substrate, and then is cooled. Also, an amorphous silicon
film formed on a glass substrate is crystallized into a crystalline
silicon film by heating and then the glass substrate is mounted on
a stage having a flat surface in such a manner that the lower
surface of the glass substrate is in close contact with the flat
surface of the stage by pressing the upper surface of the glass
substrate. Then, a linear laser beam is irradiated on the
crystalline silicon film in a scanning manner.
Inventors: |
Yamazaki, Shunpei; (Tokyo,
JP) ; Tanaka, Koichiro; (Kanagawa, JP) |
Correspondence
Address: |
FISH & RICHARDSON P.C.
P.O. BOX 1022
MINNEAPOLIS
MN
55440-1022
US
|
Assignee: |
Semiconductor Energy Laboratory Co.
Ltd., a Japan corporation
|
Family ID: |
34624004 |
Appl. No.: |
11/124053 |
Filed: |
May 9, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11124053 |
May 9, 2005 |
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09304523 |
May 4, 1999 |
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6902616 |
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09304523 |
May 4, 1999 |
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08683722 |
Jul 18, 1996 |
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5907770 |
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Current U.S.
Class: |
438/166 ;
257/E21.134; 438/479 |
Current CPC
Class: |
C30B 29/06 20130101;
H01L 21/02672 20130101; H01L 21/02532 20130101; C30B 1/00 20130101;
Y10S 117/904 20130101; Y10T 117/1004 20150115; H01L 21/02678
20130101; H01L 21/02422 20130101; H01L 29/6675 20130101; Y10T
117/1008 20150115; H01L 21/02686 20130101; B23K 26/702
20151001 |
Class at
Publication: |
438/166 ;
438/479 |
International
Class: |
H01L 021/00; C30B
001/00; H01L 021/84; H01L 021/20; H01L 021/36 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 19, 1995 |
JP |
7-205379 |
Jul 31, 1995 |
JP |
7-215406 |
Aug 4, 1995 |
JP |
7-219533 |
Aug 18, 1995 |
JP |
7-233306 |
Claims
1-21. (canceled)
22. A apparatus of manufacturing a semiconductor comprising: a
glass substrate; a stage having means for flattening and mounting
the glass substrate; and means for irradiating a linear laser beam
onto an irradiated surface over the glass substrate while scanning
the linear laser beam.
23. A laser annealing apparatus comprising: a glass substrate; a
stage having a flat surface on which the glass substrate is
mounted; means for contacting a lower surface of the glass
substrate with the flat surface of the stage; and means for
irradiating a linear laser beam onto an irradiated surface over the
glass substrate while scanning the linear laser beam.
24. A laser annealing apparatus comprising: a glass substrate
having a crystalline silicon film crystallized by heating; a stage
having means for mounting and flattening the glass substrate
thereon; and means for irradiating the linear laser beam on the
crystalline silicon film formed on the glass substrate while
scanning the linear laser beam.
25. A laser annealing apparatus comprising: a glass substrate
having a crystalline silicon film crystallized thereon by heating;
a stage having a flat surface on which the glass substrate is
mounted and means for contacting a lower surface of the glass
substrate with the flat surface thereof; and means for irradiating
a linear laser beam onto the crystalline silicon film while
scanning the linear laser beam.
26. A laser annealing apparatus comprising: a glass substrate
having a crystalline silicon film crystallized thereon by heating;
a stage having a flat surface on which the glass substrate is
mounted and means for making a lower surface of the glass substrate
suck on the flat surface thereof under vapor; and means for
irradiating the linear laser beam onto the crystalline silicon film
while scanning the linear laser beam.
27. A laser annealing apparatus comprising: a glass substrate
having a crystalline silicon film crystallized thereon by heating;
a stage having a flat surface on which the glass substrate is
mounted and means for pressing an upper surface of the glass
substrate; and means for irradiating a linear laser beam onto the
crystalline silicon film while scanning the linear laser beam.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method and an apparatus
for producing a semiconductor device which is capable of obtaining
a crystalline silicon film high in uniformity by improving the
flatness of a glass substrate during producing an insulated gate
semiconductor device such as a thin film transistor (TFT) formed
using a non-single crystal silicon film which is formed on the
substrate, or other semiconductor devices. In particularly, the
present invention is useful in producing a semiconductor device
formed on the glass substrate.
[0003] 2. Description of the Related Art
[0004] In recent years, several researches have been made into
insulated gate field effect transistors having a thin film-shaped
active layer (so-called active region) on an insulated substrate,
which are so-called TFT).
[0005] Those TFTs are classified, for example, into an amorphous
silicon TFT and a crystalline silicon TFT, depending upon the
material or the crystal state of a semiconductor as used. The
crystalline silicon stated here is directed to non-single crystal
silicon which is not single crystal. Hence, those TFTs are
generally called "non-single crystal silicon TFTs".
[0006] In general, the electric field mobility of a semiconductor
which is in an amorphous state is small, and therefore not
available to the TFTs that require a high speed operation. Also,
the amorphous silicon cannot be used to produce a p-channel TFT
(PMOS TFT) since the p-type electric field mobility of the
amorphous silicon is remarkably small, and thus a complementary MOS
circuit (CMOS) cannot be formed by the combination of the p-channel
TFT and an n-channel TFT (NMOS TFT).
[0007] The crystalline semiconductor is larger in the electric
field mobility than the amorphous semiconductor, and therefore
enables a high speed operation. The crystalline silicon can be used
for obtaining not only the NMOS TFT but also the PMOS TFT, thereby
forming a CMOS circuit.
[0008] A crystalline silicon film is obtained by thermally cooling
an amorphous silicon film obtained through the vapor phase growth
technique at an appropriate temperature (600.degree. C. or higher)
for a long period of time, or by irradiating an intense light such
as a laser beam (optically annealing).
[0009] However, in the event of using a glass substrate which is
inexpensive and rich in processability as an insulating substrate,
it is extremely difficult to obtain, by only annealing, a
crystalline silicon film which is satisfactorily high in electric
field mobility (high to the degree that the CMOS circuit can be
formed).
[0010] This is because the above glass substrate is generally low
in strain point (about 600.degree. C.), and thus when the
temperature of the substrate is elevated up to a temperature
required for obtaining a crystalline silicon film sufficiently high
in mobility, the substrate is warped.
[0011] In the event of applying the optically annealing technique
for crystallizing a silicon film formed on the glass substrate, a
high energy can be applied only to the silicon film without
elevating the temperature of the substrate so much. Hence, the
optically annealing technique is very effective in crystallizing
the silicon film formed on the glass substrate.
[0012] It has been found that a high power pulse laser such as an
excimer laser is the most suitable for the optimum light source for
optically annealing. The maximum energy of the laser is extremely
larger than that of the continuously oscillating laser such as an
argon ion laser, and the mass productivity could be enhanced using
a large spot which is several cm.sup.2 or more in size.
[0013] However, because the beam as normally used is square or
rectangular in shape, the beam is required to move vertically and
horizontally for processing a single substrate having a large area.
Thus, the optically annealing technique needed to be still improved
from the viewpoint of the productivity.
[0014] The above matter could be remarkably improved with a
technique in which the beam is deformed linearly, the width of the
beam is a length that exceeds that of a substrate to be processed,
and the substrate is scanned relatively by the beam (The scanning
operation in the specification means that linear laser beams are
irradiated onto the substrate while being overlapped one on another
bit by bit.). The details are disclosed in Japanese Patent
Unexamined Publication No. 5-112355.
[0015] The silicon film which is still high in crystallinity can be
prepared by conducting the thermally annealing process prior to
conducting the optically annealing process. In the thermally
annealing process as described in Japanese Patent Unexamined
Publication No. 6-244204, utilizing the effect that an element such
as nickel, iron, cobalt, platinum, paradium (hereinafter referred
to as "crystallized catalytic element, or simply "catalytic
element") promotes the crystallization of amorphous silicon, the
crystalline silicon film can be obtained by the thermally annealing
process at a lower temperature for a shorter period of time in
comparison with the normal case.
[0016] TFTs arranged in the form of a matrix are formed with a
crystalline silicon film formed using the thermally annealing
process and the optically annealing process together, and the
distribution of their threshold voltage in the substrate surface is
investigated.
[0017] FIG. 2 shows the distribution of the threshold values of the
TFT using the crystalline silicon film formed through the
conventional method, within the substrate surface. The distribution
is U-shaped as shown in FIG. 2.
[0018] FIG. 4 shows the arrangement of TFTs on the glass substrate.
The data in FIG. 2 is obtained, as shown in FIG. 4, in such a
manner that the TFTs of 400.times.300 pieces are arranged in the
form of a matrix in a region of 40'50 mm on a Corning 1737
substrate of 100 mm.sup.2, and the respective locations of 400 TFTs
disposed laterally in a line from one end to the other end (a
portion surrounded by a dotted line in FIG. 4) are indicated
correspondingly in the axis of abscissa.
[0019] When the pixel matrix that constitutes the pixel portion of
a liquid crystal display has the distribution of threshold voltages
shown in FIG. 2, the display state becomes nonuniform, resulting in
a defective image.
[0020] As a result of researching the cause that the threshold
voltage exhibits such a U-shaped distribution within the substrate
surface by the applicant, it has been found that a tendency of the
U-shaped distribution is very similar to the warp of the substrate
immediately before a laser beam is irradiated onto the
substrate.
[0021] Also, no warp of the substrate is found in the glass
substrate immediately after an amorphous silicon film is formed on
the glass substrate, and it has been found that the warp of the
substrate is caused because, during a heat treatment (by which the
film grows in the solid phase into a crystallized film) subsequent
to the amorphous silicon film forming process, a silicon film (or
silicon oxide film) is contracted higher than that of the glass
substrate in cooling the substrate after the heat treatment. The
warp of the substrate is produced in a U-shape viewed from the film
formed on the substrate.
[0022] FIG. 3 shows a state in which a laser annealing is conducted
on the silicon film formed on the substrate which has been warped.
From FIG. 3, when the laser annealing is conducted on the silicon
film in such a state where the substrate is warped, the focal point
of the laser beam is shifted differently at the respective
locations on the substrate.
[0023] It is presumed that the shift of the focal point makes the
crystallinity of the silicon film different from each other within
the substrate surface, so that the threshold voltage exhibits a
specified distribution within the substrate surface.
[0024] The warp of the substrate immediately before a laser beam is
irradiated onto the substrate having 100 mm square is different by
about 50 .mu.m between the central portion and the edge portion of
the substrate. The degree of the warp fell within a range of about
20 to 200 .mu.m although it depends upon the temperature of the
above heat treatment process, a time necessary for processing, the
material of the substrate, or the like. There is a case in which
when the size of the substrate is about 500 mm square, its warp
becomes about 1 to 2 mm.
SUMMARY OF THE INVENTION
[0025] The present invention has been made in view of the above
circumstances, and therefore an object of the present invention is
to enhance the flatness of a substrate after the heating process
and the annealing process have been conducted on the substrate on
which a film is formed.
[0026] Another object of the present invention is to provide a
method of producing a crystalline silicon film formed on a glass
substrate and having a uniform crystallinity within a substrate
surface. Another object of the present invention is to provide a
method of producing a plurality of crystalline silicon TFTs formed
on a glass substrate and having a uniform threshold voltage within
a substrate surface. Another object of the present invention is to
provide a method of producing, in a process of crystallizing a
silicon film on a glass substrate, in particular, having a
thermally annealing process and a laser annealing process
subsequent to the thermally annealing process, a crystalline
silicon TFT having a uniform crystallinity within a substrate
surface and also uniform threshold voltage within the substrate
surface using the silicon film.
[0027] In order to solve the above problems, the present invention
is achieved by a method of producing a semiconductor, including the
steps of forming an amorphous silicon film on a glass substrate or
on a silicon oxide film formed on the glass substrate, flattening
the glass substrate at a temperature equal to or higher than a
strain point of the glass substrate and equal to or lower than a
softening point thereof, and conducting a laser annealing process
on the silicon film.
[0028] Also, the present invention is achieved by a method of
producing a semiconductor, including the steps of forming an
amorphous silicon film on a glass substrate or on a silicon oxide
film formed on the glass substrate, flattening the glass substrate
at a temperature equal to or higher than a strain point of the
glass substrate and equal to or lower than a softening point
thereof and crystallizing the amorphous silicon film, and
conducting a laser annealing process on the silicon film which has
been crystallized through the above process.
[0029] The present invention is achieved by a method of producing a
semiconductor device, including a step of forming a plurality of
thin film transistors (TFTs) having the silicon film produced by
the above semiconductor producing method as an active layer.
[0030] As described above, in the process of producing a TFT formed
on the glass substrate, the glass substrate is warped and deformed
after the step of thermally annealing the amorphous silicon film on
the glass substrate.
[0031] Upon irradiating a laser beam onto the substrate thus
deformed, the effect of irradiation of the laser beam is different
at the respective locations of the substrate.
[0032] Therefore, a laser beam is irradiated onto the substrate
after the substrate is processed into an extreme flat state prior
to the laser irradiating step.
[0033] The present invention has been made by irradiating a laser
beam onto the substrate after the substrate before being subjected
to the laser irradiating step is subjected to a thermal treatment
into an extreme flat state.
[0034] The glass substrate on which the amorphous silicon film has
been formed is thermally annealed at a temperature equal to or
higher than the strain point (593.degree. C.) but equal to or lower
than the softening point (844.degree. C.) of the material of the
glass substrate (for example, Corning 7059), for example, at
640.degree. C. for about four hours. Thus, in the event of
subjecting the substrate to the heat treatment in advance, the
effective method is that the substrate is held at a temperature
equal to or higher than the strain point of the glass as used and
equal to or lower than the softening point thereof for several
hours, and thereafter the substrate is cooled, from the viewpoint
of the applicant's experiment. (It is difficult to flatten the
substrate because it is so hard when the temperature is less than
the strain point. The substrate is softened to the degree that the
thickness of the substrate is changed when the temperature is
higher than the softening point.)
[0035] The temperature within the temperature range, preferably a
temperature close to an annealing point (639.degree. C.) is most
preferable in flattening the substrate.
[0036] In this case, the glass substrate is located on a base
having a surface which has been flattened with high accuracy
(preferably the roughness and waviness of the surface is 5 .mu.m or
less).
[0037] The glass substrate which is thermally annealed under the
above condition is extremely lower in viscosity than that in the
state of a room temperature, and the glass substrate is brought in
close contact with the above base which is flattened with high
accuracy by self-weight.
[0038] With cooling from the close contact state, the glass
substrate is solidified while keeping that state. The glass
substrate is flattened with high accuracy.
[0039] Also, the amorphous silicon film formed on the glass
substrate is thermally annealed simultaneously in the above glass
substrate flattening step so that the film grows in solid phase.
Hence, the crystallization of the silicon film can be conducted
simultaneously when the glass substrate is flattened.
[0040] As a result that the applicant investigated the influence of
any steps for forming the TFT on the substrate on the shape of the
substrate, the deformation of the substrate after the step of
thermally annealing (including the cooling process) the amorphous
silicon film has been completed is most remarkable, and no
remarkable deformation has been found in the steps subsequent to
the thermally annealing step. Hence, if the substrate is processed
into a remarkable flat state immediately before the irradiation of
a laser beam onto the substrate, the substrate after being
subjected to all the steps can be kept in the flat state.
[0041] When a liquid crystal display is formed using the substrate,
the glass substrate can be flattened extremely excellently,
resulting in such an advantage that the cell pair can be made
readily and surely.
[0042] In general, if the roughness and waviness of the surface of
the substrate constituting the liquid crystal display is out of 5
.mu.m or less, something interferes with the cell pair. Hence, it
is extremely effective that the roughness and waviness of the
flattened base with high accuracy to be used in the present
invention, as well as the roughness and waviness of the surface of
the substrate as formed are 5 .mu.m or less.
[0043] Further, in order to solve the above problem, the present
invention is characterized in that a base on which the glass
substrate is mounted has a convex curved surface. In other words,
the present invention has been achieved by a method of producing a
semiconductor, characterized by crystallizing an amorphous silicon
film formed on a flat glass substrate by heating, locating the
glass substrate on a base having the convex curved surface, heating
the glass substrate at a temperature close to the strain point of
the glass substrate, and cooling the glass substrate.
[0044] Also, the present invention has been achieved by a method of
producing a semiconductor, including the steps of crystallizing an
amorphous silicon film formed on a flat glass substrate by heating,
locating the glass substrate on a base having the convex curved
surface, heating the glass substrate at a temperature close to the
strain point of the glass substrate, cooling the glass substrate,
and thereafter irradiating a laser beam onto the silicon film.
[0045] As described above, in the process of producing the TFT,
etc., formed on the glass substrate, the glass substrate is warped
and deformed after the step of thermally annealing the amorphous
silicon film on the glass substrate.
[0046] Upon irradiation of a laser beam onto a substrate thus
warped and deformed, the focal point of the laser beam is different
at the respective locations of the substrate, with the result that
the crystallinity becomes uniform within the substrate surface.
[0047] In view of the above, the glass substrate is made in a flat
state after the thermally annealing step, and thus a laser beam is
irradiated onto the substrate, thereby crystallizing the substrate
uniformly within the substrate surface.
[0048] FIGS. 6A to 6C show a producing method in accordance with
the present invention. A glass substrate 601 which is deformed in a
convex shape is located on a base 602 (stage) having a convex
curved surface which is substantially symmetric with the curved
surface of the substrate 601 (deformed in the convex shape) which
has been thermally annealed (thermally annealing and cooling) after
a silicon film 600 is formed on the substrate 601. The substrate
601 is deformed by heating at a temperature close to the strain
point of the glass substrate so that it is brought in close contact
with the convex curved surface of the base 602 in FIG. 6B. Then,
the substrate 601 is cooled. In cooling the substrate 601, a
silicon film 600 contracts more markedly than that of the substrate
601, with the result that the substrate 601 changes from the convex
curved surface state to the flat state as shown in FIG. 6C.
[0049] The present invention is achieved by a method of producing a
semiconductor, including the steps of locating on a base having a
convex curved surface a flat glass substrate on which an amorphous
silicon film is formed, heating the glass substrate at a
temperature close to the strain point of the glass substrate,
thereafter cooling the glass substrate.
[0050] The present invention is achieved by a method of producing a
semiconductor, including the steps of locating on a base having a
convex curved surface a flat glass substrate on which an amorphous
silicon film is formed, heating the glass substrate at a
temperature close to the strain point of the glass substrate,
cooling the glass substrate, and then irradiating a laser beam onto
the silicon film.
[0051] FIGS. 7A to 7C show a producing method in accordance with
the present invention. In crystallizing an amorphous silicon film
700 through the thermally annealing step, a glass substrate 701 is
mounted on a convex curved surface type base 702 (stage) as shown
in FIG. 7A, and the substrate is so heated as to be deformed into a
convex curved surface type.
[0052] Then, the glass substrate 701 is deformed along the convex
surface of the base 702 because of the lowering of the viscosity
due to heating and the self-weight. Keeping this state, the
substrate is heated, and then cooled after the completion of the
heat treatment.
[0053] In this state, the silicon film 700 contracts more sharply
than that of the glass substrate 701, and the glass substrate 701
returns from the convex curved surface type to the flat state (FIG.
7C).
[0054] In this way, the flattening of the glass substrate 701 and
the crystallization of the semiconductor film 700 can be conducted
simultaneously.
[0055] When the temperature necessary for the above glass substrate
flattening process is within 70 to 115% of the strain point of the
substrate, the effect of flattening the substrate is obtained.
[0056] When the heating temperature is lower than 70% of the strain
point of the substrate, the substrate is not deformed at all, or a
very long time is required for the deformation of the substrate.
When the heating temperature is more markedly than 115% of the
strain point of the substrate, the deformation of the substrate is
remarkable, and thus the shape of the substrate is not fixed after
cooling.
[0057] In the case where the glass substrate is flattened
simultaneously when the amorphous silicon film is crystallized, it
is more preferable that the temperature is higher in order to
enhance the crystallinity. However, it has been recognized that the
crystallinity is sufficiently improved even within the above
temperature range. The temperature range is a value in a case of
setting an absolute zero point as a reference.
[0058] FIG. 8 shows the producing method in accordance with the
present invention. A glass substrate 801 on which a silicon film
has been formed is located along a base 802 (stage) having a convex
curved surface by pressing the edges of the substrate 801 by
pushers 803, or the like, and the glass substrate 801 is deformed
along the convex curved surface in a state before heating is
conducted.
[0059] It is preferable that the stage 802 is made of quartz in
order to prevent the substrate 801 from being tainted.
[0060] The glass substrate 801 is heated while maintaining this
state, and the silicon film formed on the glass substrate 801 is
subjected to the laser annealing process in this state.
[0061] The heating temperature at this time is the room temperature
to 70% of the strain point of the glass substrate.
[0062] When the heating temperature exceeds 70% of the strain point
of the glass substrate 801, the glass substrate 801 is liable to be
thermally deformed, which makes it difficult to allow the substrate
801 to return to the flat surface after cooling. In the case where
the heating temperature is an excessively low temperature, that is,
lower than the room temperature, the crystallization becomes
insufficient because heat is radiated.
[0063] Then, the substrate is cooled. In cooling the substrate, the
silicon film contracts more sharply than that of the glass
substrate 801, with the result that the glass substrate 801 returns
from the convex curved surface type to the flat state.
[0064] FIG. 9 shows a substrate heating unit. When the substrate is
heated in a system of FIG. 9, heating can be efficiently conducted
on a substrate 901 having a curved surface. A base 903 having a
heater 902 is located under the substrate 901, helium gas is heated
by the heater 902, and the helium gas thus heated is circulated
under the substrate 901 by a pump 904, thereby maintaining the
substrate 901 to a desired temperature. The substrate 901 is fixed
by pushers 905. The helium gas is used because the thermal
conductivity is high.
[0065] In FIG. 10, a glass substrate 951 on which a silicon film
has been formed is pushed on a base 952 (stage) having a convex
U-shaped curved surface by pushers 953, thereby making the glass
substrate 951 curve into a convex U-shaped curved surface.
[0066] The glass substrate 951 is heated while maintaining this
state, and the silicon film formed on the glass substrate is
annealed by a laser beam in this state.
[0067] The heating temperature at this time is the room temperature
to 70% of the strain point of the glass substrate 951. A preferable
heating method is shown in FIG. 9.
[0068] When the heating temperature exceeds 70% of the strain point
of the glass substrate 951, the glass substrate 951 is liable to be
thermally deformed, which makes it difficult to allow the substrate
951 to return to the flat surface after cooling. In the case where
the heating temperature is an excessively low temperature, that is,
lower than the room temperature, the crystallization becomes
insufficient because heat is radiated.
[0069] A laser beam used for laser annealing is processed into a
linear shape for the purpose of enhancing the efficiency of a laser
processing.
[0070] FIG. 11 shows a laser irradiating method. In FIG. 11, the
height of a base (stage) indicated by a dotted line fluctuates in
accordance with the degree of a curvature of the substrate 960 in
such a manner that the focal surface of a laser beam is always
positioned on a surface to be processed.
[0071] Since the degree of the curvature of the substrate 960 is
found by the shape of the base, the thickness of the substrate,
etc., in advance, the height of the base is allowed to fluctuate on
the basis of the data, whereby the focal surface of the linear
laser beam may be kept constant regardless of the degree of
curvature of the substrate. Also, an optical system is not changed
as it is, and under the substantially same condition as in the case
of using a flat substrate, the laser annealing can be
conducted.
[0072] In order to irradiate the linear laser beam onto the curved
surface which is curved into a U-shaped type as shown in FIG. 10, a
laser beam is irradiated thereon as shown in FIG. 11, thereby being
capable of conducting a uniform laser irradiation regardless of the
substrate being curved, to thus obtain a high processing efficiency
and a high uniformity of laser annealing as in the flat
substrate.
[0073] This is a case where a linear laser beam is irradiated onto
the U-shaped curved surface. Also, in the case of conducting a
laser irradiation on the convex curved surface using a laser beam
which is not linear but square, the laser anneal can be conducted
likewise.
[0074] The focal point of the laser beam may fluctuate by not the
height of the substrate but the adjustment of a lens. However, to
make the focal point of the laser beam fluctuate, there is a case
in which there is required such an optical design that the
distribution of energy of the laser beam on the surface to be
irradiated, the depth of focus thereof, etc., is not changed.
[0075] Then, the substrate is cooled. In cooling the substrate, the
silicon film contracts more markedly than that of the glass
substrate, with the result that the glass substrate returns from
the convex curved surface type to the flat state, thereby obtaining
a flat substrate having a crystalline silicon film.
[0076] As a result that the applicant investigated the influence of
any step for forming the TFT on the substrate on the shape of the
substrate, the deformation of the substrate before and after the
step of the heat treatment for crystallizing the silicon film is
most remarkable, and no remarkable deformation has been found in
the steps subsequent to the heat treatment. Hence, if the substrate
is processed into a remarkable flat state immediately before the
irradiation of a laser beam onto the substrate, the substrate after
being subjected to all the steps can be kept in a fairly flat
state.
[0077] The producing method of the present invention provides a
crystalline silicon film having an extremely uniform crystallinity
within the substrate surface, and also provides a flat
substrate.
[0078] In the present invention, the roughness and waviness of the
glass substrate can be about 10 .mu.m or less in a substrate 1.1 mm
in thickness and 100 mm.times.100 mm in size. When the substrate is
about 500 mm in size (for example, 370.times.400 mm.sup.2,
400.times.500 mm.sup.2, 550.times.650 mm.sup.2 in size) and about
0.5 to 0.7 mm in thickness, the degree of the warp of the substrate
after thermally crystallizing and cooling the amorphous silicon
film may be 1 to 2 mm in difference in level. However, the present
invention can manufacture the substantially flat substrate.
[0079] It should be noted that the convex curved surface and the
U-shaped curved surface of the base on which the glass substrate is
mounted is determined in accordance with the size, the thickness
and the material of the glass substrate, the sort and the thickness
of the film formed on the substrate, and other various
conditions.
[0080] As the substrate is increased in area, the degree of
curvature of the substrate is increased more. Also, it is curved
two-dimensionally. Hence, in case of the glass substrate of about
100 mm.times.100 mm, the base on which the substrate is mounted may
be of the shape having a U-shaped convex curved surface which is
curved in only one direction. In this case, it is desirable that
the convex curved surface of the inverse U-shape type of the base
has 20 to 200 .mu.m, preferably about 50 .mu.m in a difference in
level between the central portion of a region of the convex curved
surface on which the glass substrate is mounted and the lowest
portion of the edge of that region.
[0081] Also, When the size of the substrate is increased to the
degree of 500 mm square, the glass substrate may be curved in two
directions. Therefore, it is preferable to use the base having the
convex curved surface such that the sections along the two
directions become the inverse U-shape type. In the case of using a
large-area glass substrate, it is desirable that a difference in
level between the central portion of a region of the convex curved
surface on which the glass substrate is mounted and the lowest
portion of the edge of that region is about 1 to 2 mm.
[0082] As a result of forming a plurality of TFTs using a
crystalline silicon film formed in accordance with the producing
method of the present invention, the distribution of the threshold
voltage of the TFTs can be made extremely uniform within the
substrate surface. This effect is increased more as the substrate
becomes large in area.
[0083] Also, the crystalline silicon TFTs for pixels and drive are
disposed on the glass substrate in accordance with the present
invention, and a liquid crystal display is formed using the
substrate. In this case, because the glass substrate can be
flattened extremely excellently in accordance with the producing
method of the present invention, there is advantageous in that the
cell pair can be made readily and surely. In this case, even when
there is no crystallizing step due to the laser irradiation after
the thermal crystallization, the effect of the invention to flatten
the substrate is effective.
[0084] The present invention is characterized in that, in order to
correct the warp of the glass substrate flatly, the glass substrate
is sucked on a flat surface of a stage having the flat surface, the
peripheral portion of the glass substrate is pressed (pressurized),
or the like. Also, the present invention is characterized in that
such a flattening and correcting means is added to the stage of the
laser annealing unit.
[0085] According to the present invention, there is provided a
laser annealing method including the steps of flattening a glass
substrate on a stage and mounting the former on the latter, and
irradiating a linear laser beam on the surface to be irradiated on
the glass substrate in a scanning manner to conduct the laser
annealing.
[0086] According to the present invention, there is provided a
laser annealing method including the steps of crystallizing an
amorphous silicon film formed on a glass substrate by heating into
a crystalline silicon film, flattening the glass substrate on the
stage and mounting the former on the latter, and irradiating a
linear laser beam on the crystalline silicon film in a scanning
manner to conduct the laser annealing.
[0087] According to the present invention, there is provided a
laser annealing method including the steps of crystallizing an
amorphous silicon film formed on a glass substrate by heating into
a crystalline silicon film, mounting the glass substrate on a stage
having a flat surface in such a manner that the lower surface of
the glass substrate is in close contact with the flat surface of
the stage, and irradiating a linear laser beam on the crystalline
silicon film in a scanning manner to conduct the laser
annealing.
[0088] According to the present invention, there is provided a
laser annealing method including the steps of crystallizing an
amorphous silicon film formed on a glass substrate by heating into
a crystalline silicon film, mounting the glass substrate on a stage
having a flat surface in such a manner that the lower surface of
the glass substrate is sucked with the flat surface of the stage
under vapor, and irradiating a linear laser beam on the crystalline
silicon film in a scanning manner to conduct the laser
annealing.
[0089] According to the present invention, there is provided a
laser annealing method including the steps of crystallizing an
amorphous silicon film formed on a glass substrate by heating into
a crystalline silicon film, mounting the glass substrate on a stage
having a flat surface in such a manner that the lower surface of
the glass substrate is in close contact with the flat surface of
the stage by pressing the peripheral portion of the upper surface
of the glass substrate, and irradiating a linear laser beam on the
crystalline silicon film in a scanning manner to conduct the laser
annealing.
[0090] According to the present invention, there is provided a
laser annealing device including a stage having means for
flattening and mounting a glass substrate, and means for
irradiating a linear laser beam onto a surface to be irradiated of
the glass substrate in a scanning manner.
[0091] According to the present invention, there is provided a
laser annealing device including a stage having a flat surface on
which a glass substrate is mounted and means for contacting the
lower surface of the glass substrate with the flat surface thereof,
and means for irradiating a linear laser beam onto a surface to be
irradiated of the glass substrate in a scanning manner.
[0092] According to the present invention, there is provided a
laser annealing device including a stage having means for
flattening and mounting a glass substrate having thereon a
crystalline silicon film which has been crystallized by heating,
and means for irradiating a linear laser beam on a crystalline
silicon film on the glass substrate in a scanning manner.
[0093] According to the present invention, there is provided a
laser annealing device including a stage having a flat surface on
which a glass substrate having a crystalline silicon film
crystallized by heating is mounted and means for contacting the
lower surface of the glass substrate with the flat surface thereof,
and means for irradiating a linear laser beam onto the crystalline
silicon film in a scanning manner.
[0094] According to the present invention, there is provided a
laser annealing device including a stage having a flat surface on
which a glass substrate having a crystalline silicon film
crystallized thereon by heating is mounted and means for making the
lower surface of the glass substrate suck on the flat surface
thereof under vapor, and means for irradiating a linear laser beam
onto the crystalline silicon film in a scanning manner.
[0095] According to the present invention, there is provided a
laser annealing device including a stage having a flat surface on
which a glass substrate having a crystalline silicon film
crystallized thereon by heating is mounted and means for pressing
the peripheral portion of the upper surface of the glass substrate,
and means for irradiating a linear laser beam onto the crystalline
silicon film in a scanning manner.
[0096] According to the present invention, in the above structure,
the crystalline silicon film may be a film a part of which contains
an impurity by ion doping or the like.
[0097] Also, according to the present invention, in the above
structure, a pulse laser, more preferably an excimer laser may be
used as a light source of the linear laser beam.
[0098] The present invention is that when the crystalline silicon
film obtained by thermally crystallizing the amorphous silicon film
formed on the glass substrate, patterned, machined and shaped
crystalline silicon film, or those crystalline silicon film being
added with an impurity is subjected to a laser annealing by
scanning the linear laser beam, the warp of the glass substrate
caused by the thermally annealing step is annealed on the stage on
which the glass substrate is mounted by a laser beam in a forcedly
flattened state.
[0099] In the present invention, the flattening of the glass
substrate is to correct the glass substrate in such a manner that
some external force is applied to the glass substrate to reduce the
warp of the substrate in the state where the glass substrate is
located on the stage.
[0100] The flattening of the glass substrate is conducted to the
degree that the crystalline silicon film on the glass substrate is
formed can be uniformly annealed using a linear laser beam.
[0101] A difference in level within the surface of the crystalline
silicon film on the glass substrate may be reduced so that the
crystallinity of the crystalline silicon film falls within a range
which unifies into a required level after the laser annealing.
[0102] In the present invention, with the glass substrate being
mounted flatly, the linear laser beam is irradiated uniformly onto
the crystalline silicon film which is a surface to be irradiated
without any displacement of the focal point regardless of the glass
substrate per se being curved.
[0103] As a result, the crystalline silicon film having the uniform
crystallinity and mobility with the same quality within the
substrate surface can be obtained.
[0104] According to the present invention, the crystalline silicon
film on the glass substrate which has been warped after the
thermally crystallization has been conducted is corrected to the
degree that the warp of the glass substrate can be ignored, thereby
irradiating a linear laser beam thereon.
[0105] Therefore, the focal point of the linear laser beam on the
surface to be irradiated can be prevented from shifting. As a
result, even though the laser annealing is conducted on the
substrate by scanning the linear laser beam, the uniform
crystallization can be conducted, and the threshold voltage of the
TFT on which the film is formed can be unified within the substrate
surface.
[0106] Because the degree of warp is more remarkable as the glass
substrate is increased in size, the effect of the present invention
becomes more remarkable as the glass substrate is increased in
size.
BRIEF DESCRIPTION OF THE DRAWINGS
[0107] FIGS. 1A to 1F show a producing process in accordance with
an embodiment of the present invention;
[0108] FIG. 2 shows the distribution of the threshold voltage of a
TFT using a crystalline silicon film formed in accordance with a
conventional method within a substrate surface;
[0109] FIG. 3 shows a state in which a silicon film formed on the
glass substrate which has been warped is annealed by a laser
beam;
[0110] FIG. 4 shows a TFT formed on the glass substrate;
[0111] FIG. 5 shows the distribution of the threshold voltage of a
TFT using a crystalline silicon film formed in accordance with the
embodiment within a substrate surface;
[0112] FIGS. 6A to 6C show the producing method of the
embodiment;
[0113] FIGS. 7A to 7C show the producing method of the
embodiment;
[0114] FIG. 8 shows the producing method of the embodiment;
[0115] FIG. 9 shows a substrate heating unit;
[0116] FIG. 10 shows the producing method of the embodiment;
[0117] FIG. 11 shows a laser irradiating method;
[0118] FIG. 12 is a conceptual diagram showing a laser annealing
unit used in the embodiment;
[0119] FIGS. 13A to 13C show an optical system;
[0120] FIGS. 14A and 14B show an optical system;
[0121] FIGS. 15A and 15B show the distribution of energy of a laser
beam;
[0122] FIG. 16 show the distribution of the energy density of a
laser beam processed into a linear shape in the widthwise direction
of the laser beam;
[0123] FIGS. 17A to 17D are structural diagrams showing another
stage; and
[0124] FIGS. 18A to 18C show a laser irradiating process.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
First Embodiment
[0125] FIGS. 1A to 1F show a process of producing a thin film
transistor (TFT) in accordance with a first embodiment.
[0126] An under silicon oxide film 102 having a thickness of 2000
.ANG. is formed on a glass substrate (in this embodiment, using a
Corning 7059 of 100 mm square), and an amorphous silicon film 103
having a thickness of 500 .ANG. is formed sequentially on the under
silicon oxide film 102 by plasma CVD.
[0127] Then, in order to add nickel as a catalytic element that
promotes crystallization, nickel acetate aqueous solution of 10 ppm
is coated on the surface of silicon, and nickel acetate layer not
shown is formed through the spin coating technique.
[0128] It is more preferable that a surface active agent is added
to the nickel acetate aqueous solution. Since the nickel acetate
layer is very thin, although it is not limited to a film shape, it
does not suffer from a problem in the subsequent process (FIG.
1A).
[0129] Then, the glass substrate 101 is located on a stage having a
surface flattened with high accuracy (the roughness and waviness of
the surface is 5 .mu.m or less), and then thermally annealed at
640.degree. C. for four hours, to flatten glass substrate 101 and
crystallize the amorphous silicon film. In this state, nickel
serves as the core of crystal, to promote the crystallization of
the amorphous silicon film.
[0130] The strain point of the Corning 7059 substrate is
593.degree. C., and the softening point is 844.degree. C., and the
annealing temperature of 640.degree. C. is between the strain point
and the softening point. Also, the annealing point of the Corning
7059 is 639.degree. C.
[0131] In thermally crystallizing the substrate, that the
processing can be made at a low temperature for a short period of
time, that is, at 640.degree. C. for four hours is caused by the
function of nickel. The details are disclosed in Japanese Patent
Unexamined Publication No. 6-244104.
[0132] The above publication discloses that the thermal annealing,
for example, at 550.degree. C. (below the strain point) for four
hours is conducted so that the temperature in thermally annealing
does not exceed the strain point of the glass substrate. However,
the temperature is determined so that the glass substrate is
deformed as little as possible in thermally crystallizing.
[0133] The present invention is to raise the substrate temperature
up to a temperature at which the glass substrate is liable to be
deformed as much as possible reversely so that the crystallization
as well as the flattening of the substrate is conducted
simultaneously.
[0134] It is preferable that the concentration of a catalytic
element is 1.times.10.sup.15 to 1.times.10.sup.19 atoms/cm.sup.3.
When it is a high concentration equal to or more than
1.times.10.sup.19 atoms/cm.sup.3, a metallic nature is exhibited on
silicon so that the semiconductor characteristic disappears. The
concentration of the catalytic element in the silicon film in this
embodiment is 1.times.10.sup.17 to 5.times.10.sup.18 atoms/cm.sup.3
at the minimum in the film. Those values are the minimum values of
the concentration of the catalytic elements in the silicon film
which has been analyzed and measured by the secondary ion mass
spectrometry (SIMS).
[0135] In this way, the crystallization of the silicon film and the
flattening of the substrate are conducted, and after the completion
of the process, the substrate is cooled up to the room temperature
at 2.degree. C./min.
[0136] In the case where the catalytic element that promotes
crystallization is not added to the silicon film, if the heating
temperature is low, then there is a case in which only the
flattening of the substrate is conducted in the above process, and
no crystallization is conducted. However, that the uniform
crystallization can be conducted in the subsequent laser annealing
process is identical with the case in which the catalytic elements
are added to the silicon film.
[0137] In order to further enhance the crystallinity of the
crystalline silicon film, an excimer laser beam which is a
high-power pulse laser is irradiated onto the film.
[0138] In this embodiment, a KrF excimer laser (248 nm in
wavelength and 30 nsec in pulse width) is processed into a linear
shape before being used. The size of a laser beam is 1.times.125
mm.sup.2. The irradiation of the laser beam is conducted with the
energy density of the laser beam being within 100 mJ/cm.sup.2 to
500 mJ/cm.sup.2, for example, 370 mJ/cm.sup.2.
[0139] The crystallinity is further enhanced by irradiating a laser
beam onto the film with an energy of about 220 mJ/cm.sup.2 in
advance before the above irradiation of the laser beam.
[0140] The laser irradiating method is as follows. The linear laser
beam is irradiated onto the film while being shifted relatively
with respect to an object to be irradiated. A direction in which
the linear laser beam is shifted is substantially perpendicular to
the line direction. In this situation, paying an attention to one
point of a surface to be irradiated, the laser beams of 2 to 20
shots are irradiated onto the point. Also, the substrate
temperature at the time of irradiating the laser beam is
200.degree. C. (FIG. 1B).
[0141] A TFT is fabricated on the crystalline silicon film 104. The
TFT is disposed on the substrate in the form of a matrix.
Specifically, TFTs of 400.times.300 pieces are fabricated in a
fabrication area of 40.times.50 mm.sup.2. This process will be
described below.
[0142] A silicon film is so etched as to form an island silicon
region 105. Then, a silicon oxide film 106 having a thickness of
1200 .ANG. is formed as a gate insulating film by plasma CVD. The
raw material gas of the plasma CVD as used is TEOS and oxygen. The
substrate temperature when forming the film on the substrate is 250
to 380.degree. C., for example, 300.degree. C. (FIG. 1C).
[0143] Sequentially, an aluminum film (containing silicon of 0.1 to
2% therein) having a thickness of from 3000 to 8000 .ANG., for
example, 600 .ANG. is deposited by sputtering. Then, the aluminum
film is so etched as to form a gate electrode 107 (FIG. 1C).
[0144] Then, an impurity (boron) is implanted into a silicon region
with a mask of the gate electrode 109 by ion doping. The doping gas
as used is diborane (B.sub.2H.sub.6) which has been diluted with
hydrogen into 1 to 10%, for example, 5%.
[0145] An accelerating voltage is 60 to 90 kV, for example, 65 kV,
the dose amount is 2.times.10.sup.15 to 5.times.10.sup.15
atoms/cm.sup.2, for example, 3.times.10.sup.15 atoms/cm.sup.2. The
substrate temperature at the time of ion doping is the room
temperature. As a result, p-type impurity regions 108 (source) and
109 (drain) are formed (FIG. 1D).
[0146] Then, in order to activate doped boron, annealing is
optically conducted again using the KrF excimer laser. The energy
density of the laser beam is 100 to 350 mJ/cm.sup.2, for example,
250 mJ/cm.sup.2. The crystallinity is further enhanced by
irradiating a laser beam onto the film with an energy of about 170
mJ/cm.sup.2 in advance before the irradiation of the laser
beam.
[0147] The laser irradiating method is as follows. The linear laser
beam is irradiated onto the film while being shifted relatively
with respect to an object to be irradiated. A direction in which
the linear laser beam is shifted is substantially perpendicular to
the line direction. In this situation, paying an attention to one
point of a surface to be irradiated, the laser beams of 2 to 20
shots are irradiated onto the point. Also, the substrate
temperature at the time of irradiating the laser beam is
200.degree. C. Thereafter, annealing is thermally conducted in the
nitrogen atmosphere at 450.degree. C. for 2 hours (FIG. 1E).
[0148] A silicon oxide film 110 having a thickness of 6000 .ANG. is
formed as an interlayer insulator by plasma CVD, in which contact
holes are defined. Then, electrodes-wirings 111 and 112 of the
source and the drain of the TFT are formed of a multi-layer film
made of a metal material, for example, titanium and aluminum.
Finally, annealing is thermally conducted at 200 to 350.degree. C.
under the hydrogen atmosphere of 1 atmospheric pressure (FIG.
1F).
[0149] FIG. 5 shows the distribution of the threshold values of the
TFT using a crystalline silicon film formed in accordance with this
embodiment within the substrate surface.
[0150] The axis of abscissa in FIG. 5 corresponds to the respective
locations of the TFT shown in FIG. 4 (a portion surrounded by a
dotted line in FIG. 4) as in the case of FIG. 2.
[0151] In FIG. 5, the TFT fabricated by this embodiment has a
uniform threshold value within the substrate surface. It is
apparent that the TFT in FIG. 5 has, within the substrate surface,
a uniform threshold voltage more than the conventional example
shown in FIG. 2.
Second Embodiment
[0152] A second embodiment will be described with reference to
FIGS. 1A to 1F. An under silicon oxide film 102 having a thickness
of 2000 .ANG. is formed on a glass substrate 101 (In this
embodiment, there is used a Corning 1737 of 400.times.500 mm square
and 0.7 mm thickness. Another glass substrate such as Corning 7059,
OA2, NA45, etc., may be used.), and an amorphous silicon film 103
having a thickness of 500 .ANG. is formed sequentially on the under
silicon oxide film 102 by plasma CVD.
[0153] Then, nickel acetate aqueous solution of 10 ppm is coated on
the surface of silicon, and a nickel acetate layer not shown is
formed by spin coating.
[0154] It is more preferable that a surface active agent is added
to the nickel acetate aqueous solution. Since the nickel acetate
layer is very thin, although it is not necessarily to be in a film
shape, it does not suffer from any problem in the subsequent
process (FIG. 1A).
[0155] Then, the glass substrate 101 is thermally annealed at
550.degree. C. for four hours, to crystallize the amorphous silicon
film 103. In this state, nickel serves as the core of crystal, to
promote the crystallization of the amorphous silicon film. The
strain point of the Corning 1737 substrate is 667.degree. C., and
the annealing temperature of 550.degree. C. is under the strain
point.
[0156] After the thermal crystallization, when the glass substrate
is cooled, the silicon film contracts so that the substrate
produces the concave warp.
[0157] That the processing can be made at a low temperature (the
strain point of the Corning 1737 or less) for a short period of
time, that is, at 550.degree. C. for four hours is caused by the
function of nickel. The details are disclosed in Japanese Patent
Unexamined Publication No. 6-244104. The publication discloses that
the thermal annealing, for example, at 550.degree. C. (below the
strain point) for four hours is conducted so that the temperature
in thermally annealing does not exceed the strain point of the
glass substrate. However, the temperature is determined so that the
glass substrate 101 is prevented from being remarkably deformed in
thermally crystallizing.
[0158] In order to correct the warp of the glass substrate 101
after the above thermally crystallizing process, the glass
substrate 601 is put on a base 602 having a convex curved surface
as shown in FIG. 6A, and then appropriately heated (at 350 to
600.degree. C. for several hours). The convex curved surface has a
curved surface which is substantially symmetric with the warp of
the glass substrate.
[0159] Then, in FIG. 6B, the glass substrate 601 is deformed along
the shape of the base 602 by self-weight and heat. In this state,
when the glass substrate is cooled, the silicon film 600 formed on
the substrate contracts more markedly than the glass substrate 601,
with the result that an extremely flat glass substrate 601 can be
obtained.
[0160] In order to further enhance the crystallinity of the
crystalline silicon film thus obtained, an excimer laser beam which
is a high-power pulse laser is irradiated onto the film.
[0161] The outline of the laser annealing unit will be described
below.
[0162] FIG. 12 shows a conceptual diagram of a laser annealing unit
used in this embodiment of the present invention. The laser
annealing unit shown in FIG. 12 is of the multi-chamber system
which is designed in such a manner that a substrate taken in from a
loader/unloader chamber 11 and positioned at an alignment chamber
12 is transferred to the respective chambers 13 through a transfer
chamber 13 by a substrate transfer robot 14 disposed in the
transfer chamber 13 so that each substrate is sequentially
processed.
[0163] The substrate is firstly taken in the thermal chamber 15,
and after being subjected to the heat treatment such as the
preliminary heating, a laser annealing is conducted in the laser
annealing chamber. Then, after the substrate is conveyed to the
cooling chamber 17 and cooled, it is moved to the loader/unloader
chamber 11, and moved outward.
[0164] The dispersion of the energy for each pulse of the laser
annealing unit is 3.sigma. within .+-.3%.
[0165] A pulse laser larger in dispersion than the above laser may
be used, but the focal depth is narrowed the dispersion of 3.sigma.
exceeding .+-.10% is not applicable to the present invention.
[0166] An oscillator as used is EX748 made by LUMNICS Corporation.
The laser beam as oscillated is a KrF excimer laser beam (248 nm in
wavelength and 25 ns in pulse width).
[0167] Another excimer laser as well as another type laser can be
used. It should be noted that a laser beam of a pulse oscillator
need to be used.
[0168] The laser annealing unit has a sealing property against the
surroundings so as to prevent the contamination due to an impurity.
Also, it has an atmosphere control function when irradiating a
laser beam. Also, the laser annealing unit has a function for
heating the substrate so that an object to be irradiated when
irradiating a laser beam can be kept to a desired temperature.
[0169] The oscillated laser beam is introduced into an optical
system as shown in FIGS. 13A and 13B because the shape of the laser
beam is deformed.
[0170] The laser beam immediately before being shone onto the
optical system is of the rectangular of about 3.times.2 cm.sup.2,
however the laser beam is processed into a slender beam (linear
beam) of about 10 to 30 cm in length and 0.01 to 0.3 cm in width,
depending on the optical system.
[0171] Also, the distribution of the energy density of the linear
laser beam that has passed through the optical system in the cross
direction is trapezoidal as shown in FIG. 15B. The energy of the
laser beam that has passed through the optical system is 800
mJ/shot at the maximum.
[0172] The reason why the laser beam is processed into such a
slender beam is to improve the processability. When the linear beam
is irradiated onto an sample, if the length of the laser beam is
longer than the width of the sample, then the sample is moved in
one direction, thereby irradiating a laser beam onto the entire
sample.
[0173] Even though the length of the beam is shorter than the width
of the sample, troublesomeness in processing is saved in comparison
with the rectangular beam. However, in this case, there occurs the
necessity of moving the beam vertically and horizontally relatively
with respect to the sample.
[0174] The stage (base) for the substrate (sample) on which a laser
beam is irradiated is controlled by a computer and so designed as
to be moved perpendicularly to the line direction of the linear
laser beam. Also, it is so designed that the height of the
substrate can fluctuate.
[0175] If the stage is provided with a function for moving in the
line direction of the laser beam, even though the beam width is
shorter than the sample, a laser processing on the entire sample is
enabled.
[0176] The optical path in the interior of the optical system that
processes a laser beam into a linear laser beam will be
described.
[0177] The laser beam incident to the optical system passes through
a cylindrical concave lens B, a cylindrical convex lens C (the
lenses B and C are generally called "beam expander"), and fly eye
lenses D and D2.
[0178] The laser beam passes through a cylindrical convex lens E as
a first cylindrical lens, and a cylindrical convex lens F as a
second cylindrical lens provided for improving the uniformity of
the linear beam in the line direction, and is then converged by the
cylindrical convex lens H via a mirror G before being irradiated
onto the surface to be irradiated.
[0179] A distance between the cylindrical lenses A and B is 230 mm,
a distance between the fly eye lenses D and D2 is 230 mm, a
distance between the fly eye lens D and the cylindrical lens E is
650 mm, and a distance between the cylindrical lens F and the
surface to be irradiated is 650 mm (the sum of the focal distances
of the respective lenses). It is needless to say that these
distances can be changed in accordance with the circumstances. The
cylindrical lens H as used has a focal distance of 120 mm.
[0180] The shape of the energy distribution of the focal points of
the laser beam is made trapezoidal by changing the lens H
vertically (J-direction).
[0181] The surface to be irradiated is moved vertically
(J-direction) relatively with respect to the lens H, thereby being
capable of deforming the shape of the energy distribution of the
laser beam on the surface to be irradiated (focal point) with a
range of from a nearly square shape to a nearly trapezoidal shape
(refer to FIG. 13C). In order to more sharpen those shapes, a slit
may be inserted in the laser optical path.
[0182] The optical system is not particularly limited if it is so
designed as to deform the laser beam to the shape required by the
present invention.
[0183] The laser beam is shaped into a linear form, and the area of
a laser beam on the surface to be irradiated is 125 mm.times.1 mm.
The width of the linear laser beam is half width of the maximum
energy value of the laser beam.
[0184] The energy profile (energy distribution) of the linear laser
beam in the cross direction has an artificial trapezoidal
distribution of L1=0.4 mm and L2, L3=0.25 mm as shown in FIG. 15B,
which satisfies the inequality of 0.5L1.ltoreq.L2.ltoreq.L1,
0.5L.ltoreq.L3.ltoreq.L1. In this case, the focal depth can be
about .+-.400 .mu.m.
[0185] The degree of widening of the bottom of the trapezoidal
distribution is changed in accordance with a distance between the
final lens of the laser optical system and the surface to be
irradiated. A distance between the final lens of the laser optical
system and the surface to be irradiated is changed by the roughness
of the object to be irradiated during the laser processing.
[0186] With any change of the distance between the final lens and
the surface to be irradiated, the degree of widening of the bottom
of the trapezoidal distribution of the laser beam is changed. If a
range of the change is within a range of the above inequality, then
the focal depth of about .+-.400 .mu.m is obtained, and therefore
when the roughness of the surface to be irradiated is .+-.400 .mu.m
or less, the uniform laser processing is enabled.
[0187] On the contrary, the general laser beam having a square
energy distribution is about +200 .mu.m or less in focal depth, and
is adversely affected by the roughness of the surface to be
irradiated and a difference in level, thereby being liable to make
the crystallinity within the substrate surface nonuniform.
[0188] The sample is put on the stage (base) within the laser
annealing chamber 16 shown in FIG. 12, and a laser beam is
irradiated while the stage is moving at 2 mm/s. The laser
irradiation conditions are that the energy density of the laser
beam is 100 to 500 mJ/cm.sup.2, in this example, 300 mJ/cm.sup.2,
and the number of pulses is 30 pulses/s. The energy density means
the density of the upper bottom portion (a portion having a maximum
value) of the trapezoidal laser beam. The substrate temperature
when irradiating the laser beam is 200.degree. C.
[0189] The irradiation of the laser beam is conducted under the
above conditions. Paying an attention to a certain point of the
sample, the laser beams are irradiated at 15 steps. This is
because, since it takes 0.5 sec to allow one laser beam to pass, 15
pulses are irradiated on one location by irradiating the laser beam
in the scanning manner. In this example, in the above 15
irradiations, the initial several irradiations have the irradiated
energy density gradually increased, and the final several
irradiations have the irradiated energy density gradually
decreased.
[0190] This state is schematically shown in FIG. 16. In the first
half of 15 steps, the laser energy gradually increases (see A in
FIG. 16) whereas, in the latter half thereof, it gradually
decreases (see B in FIG. 16).
[0191] With such an irradiation of the laser beam, the use of a
single pulse laser beam can provide the same effect as the
conventional two-step irradiation system using a weak pulse laser
beam for preliminary heating and a strong pulse laser beam for
crystallization.
[0192] Since the energy applied to a region to be irradiated is not
rapidly changed, a phase is not rapidly changed in the silicon
film, and the roughness of the surface, the storage of the internal
stress, etc., are prevented, thereby obtaining a uniform
crystallinity.
[0193] Also, in this example, the atmospheric control is not
particularly conducted, and the irradiation of the laser beam is
conducted in the atmosphere. It may be conducted under the vacuum
or the atmosphere of an inactive gas such as argon or helium,
hydrogen or nitrogen (FIG. 1B).
[0194] Then, a TFT is fabricated as a semiconductor device using
the crystalline silicon film thus fabricated in accordance with the
producing process as in the first embodiment. The TFT is arranged
in the form of a matrix on the substrate. Specifically, TFTs of
400.times.300 pieces are fabricated in a producing area of
40.times.50 mm.sup.2.
[0195] The distribution of the threshold values of the TFTs using
the crystalline silicon film formed in accordance with this
embodiment within the substrate surface is uniform as shown in FIG.
5 as in the first embodiment.
Third Embodiment
[0196] Although in the second embodiment, the glass substrate 101
of 400.times.500 mm square is used, in a third embodiment, Corning
7059 of 100 mm square is used for a glass substrate. Hence, in
flattening the glass substrate which has been subjected to the
crystallizing process, the shape of a stage on which the glass
substrate shown in FIG. 6A is mounted may be of the inverse U-shape
type convex curved surface which is curved in one direction.
[0197] The glass substrate is put on the stage having the inverse
U-shape type convex curved surface, and an appropriate heat is
applied to the glass substrate. Then, the glass substrate is
deformed along the stage by self-weight and heat. In this
situation, when the glass substrate is cooled, the silicon film
formed on the substrate contracts more sharply than the glass
substrate, with the result that an extremely flat glass substrate
can be obtained.
[0198] Thereafter, a TFT is fabricated in the same manner as that
of the first embodiment.
[0199] The distribution of the threshold voltage of the TFT is
extremely uniform within the substrate surface in comparison with
the TFT manufactured without flattening the glass substrate, as in
the first embodiment.
Fourth Embodiment
[0200] A fourth embodiment will be described with reference to
FIGS. 1A to 1F.
[0201] An under silicon oxide film 102 having a thickness of 2000
.ANG. is formed on a glass substrate 101 (In this embodiment, there
is used a Corning 1737 of 400.times.500 mm square and 0.7 mm
thickness. It is needless to say that another glass substrate such
as Corning 7059, OA2, NA45, etc., may be used.), and an amorphous
silicon film 103 having a thickness of 500 .ANG. is sequentially
formed on the under silicon oxide film 102 by plasma CVD.
[0202] Then, nickel acetate aqueous solution of 10 ppm is coated on
the surface of silicon, and a nickel acetate layer is formed by
spin coating. It is more preferable that a surface active agent is
added to the nickel acetate aqueous solution. Since the nickel
acetate layer is very thin, although it is not necessarily to be in
a film shape, it does not suffer from any problem in the subsequent
process (FIG. 1A).
[0203] Then, the glass substrate is located on the convex stage
(the rising portion in the center of a region on which the
substrate is mounted is higher in level than the edge of that
region), and then thermally annealed at 550.degree. C. for four
hours, to crystallize the amorphous silicon film.
[0204] In this situation, the glass substrate is deformed along the
stage by self-weight and heat.
[0205] Also, in this state, nickel serves as the core of crystal,
to thereby promote the crystallization of the silicon film. It
should be noted that the strain point of the Corning 1737 substrate
is 667.degree. C., and the annealing temperature of 550.degree. C.
is below the strain point.
[0206] That the processing can be made at a low temperature (the
strain point of the Corning 1737 or less) for a short period of
time, that is, at 550.degree. C. for four hours is caused by the
function of nickel. The details are disclosed in Japanese Patent
Unexamined Publication No. 6-244104. The publication discloses that
the thermal annealing, for example, at 550.degree. C. (below the
strain point) for four hours is conducted so that the temperature
in thermally annealing does not exceed the strain point of the
glass substrate. However, the temperature is determined so that the
glass substrate is prevented from being remarkably deformed in
thermally crystallizing.
[0207] It is preferable that the concentration of a catalytic
element is 1.times.10.sup.15 to 1.times.10.sup.19 atoms/cm.sup.3.
When it is a high concentration equal to or more than
1.times.10.sup.19 atoms/cm.sup.3, a metallic nature is exhibited on
silicon, whereby the semiconductor characteristic disappears. The
concentration of the catalytic element in the silicon film in this
embodiment is 1.times.10.sup.17 to 5.times.10.sup.18 atoms/cm.sup.3
at the minimum in the film. It should be noted that those values
are the minimum values of the concentration of the catalytic
elements in the silicon film which has been analyzed and measured
by the secondary ion mass spectrometry (SIMS).
[0208] After the thermal crystallization, when the glass substrate
is cooled, the glass substrate is flattened because its coefficient
of contraction is larger than that of the glass substrate.
[0209] In order to further enhance the crystallinity of the
crystalline silicon film thus obtained, an excimer laser beam which
is a high-power pulse laser is irradiated onto the film. A KrF
excimer laser (248 nm in wavelength and 30 nsec in pulse width) is
processed into a linear shape before being used. The size of a
laser beam is 1.times.125 mm.sup.2. The irradiation of the laser
beam is conducted with the energy density of the laser beam being
within 100 mJ/cm.sup.2 to 500 mJ/cm.sup.2, for example, 370
mJ/cm.sup.2. The crystallinity is further enhanced by irradiating a
laser beam onto the film with an energy of about 220 mJ/cm.sup.2 in
advance before the above irradiation of the laser beam.
[0210] The laser irradiating method is as follows. The linear laser
beam is irradiated onto the film while being shifted relatively
with respect to an object to be irradiated. A direction in which
the linear laser beam is shifted is substantially perpendicular to
the linear laser. In this situation, paying an attention to one
point of a surface to be irradiated, the laser beams of 2 to 20
shots are irradiated onto the point. Also, the substrate
temperature at the time of irradiating the laser beam is
200.degree. C. (FIG. 1B).
[0211] Thereafter, a TFT is fabricated in the same manner as that
of the first embodiment.
[0212] The distribution of the threshold voltage of the TFT thus
obtained is extremely unified within the substrate surface in
comparison with the TFT manufactured without flattening the glass
substrate, as in the first embodiment.
[0213] Also, although in this embodiment, the glass substrate 101
of 400 mm.times.500 mm square is used, in the case of using Corning
7059 of 100 mm square as the glass substrate 101 as in the third
embodiment, the shape of the stage on which the glass substrate
shown in FIG. 6A is mounted may be changed to the inverse U-shape
type convex curved surface which is curved in one direction, in
flattening the glass substrate which has been crystallized.
Fifth Embodiment
[0214] A fifth embodiment will be described with FIGS. 1A to 1F. An
under silicon oxide film 102 having a thickness of 2000 .ANG. is
formed on a glass substrate 101 (In this embodiment, there is used
a Corning 1737 of 400.times.500 mm square and 0.7 mm thickness.
Another glass substrate such as Corning 7059, OA2, NA45, etc., may
be used.), and an amorphous silicon film 103 having a thickness of
500 .ANG. is then formed on the under silicon oxide film 102 by
plasma CVD. Then, nickel acetate aqueous solution of 10 ppm is
coated on the surface of silicon, and a nickel acetate layer is
formed by spin coating. It is more preferable that a surface active
agent is added to the nickel acetate aqueous solution. Since the
nickel acetate layer is very thin, although it is not necessarily
to be in a film shape, it does not suffer from any problem in the
subsequent process (FIG. 1A).
[0215] Then, the glass substrate is thermally annealed at
550.degree. C. for four hours, to crystallize the amorphous silicon
film. In this state, nickel serves as the core of crystal, to
promote the crystallization of the silicon film. The strain point
of the Corning 1737 substrate is 667.degree. C., and the annealing
temperature of 550.degree. C. is below the strain point.
[0216] After the thermal crystallization, when the glass substrate
is cooled, the silicon film contracts, to thereby warp the glass
substrate in the form of a concave.
[0217] That the processing can be made at a low temperature (the
strain point of the Corning 1737 or less) for a short period of
time, that is, at 550.degree. C. for four hours is caused by the
function of nickel. The details are disclosed in Japanese Patent
Unexamined Publication No. 6-244104. The publication discloses that
the thermal annealing, for example, at 550.degree. C. (below the
strain point) for four hours is conducted so that the temperature
in thermally annealing does not exceed the strain point of the
glass substrate. However, the temperature is determined so that the
glass substrate is prevented from being remarkably deformed in
thermally crystallizing.
[0218] It is preferable that the concentration of a catalytic
element is 1.times.10.sup.15 to 1.times.10.sup.19 atoms/cm.sup.3.
When it is a high concentration equal to or more than
1.times.10.sup.19 atoms/cm.sup.3, a metallic nature is exhibited on
silicon, whereby the semiconductor characteristic disappears. The
concentration of the catalytic element in the silicon film in this
embodiment is 1.times.10.sup.17 to 5.times.10.sup.18 atoms/cm.sup.3
at the minimum in the film. It should be noted that those values
are the minimum values of the concentration of the catalytic
elements in the silicon film which has been analyzed and measured
by SIMS.
[0219] In order to further enhance the crystallinity of the
crystalline silicon film thus obtained, an excimer laser beam which
is a high-power pulse laser is irradiated onto the film while
heating the film. In this situation, the glass substrate which has
been warped in the form of a concave is flattened
simultaneously.
[0220] In this embodiment, a KrF excimer laser (248 nm in
wavelength and 30 nsec in pulse width) is used. The size of a laser
beam is 30.times.20 mm.sup.2. The irradiation of the laser beam is
conducted with the energy density of the laser beam being within
100 mJ/cm.sup.2 to 500 mJ/cm.sup.2, for example, 370 mJ/cm.sup.2.
The crystallinity is further enhanced by irradiating a laser beam
onto the film with an energy of about 220 mJ/cm.sup.2 in advance
before the above irradiation of the laser beam.
[0221] The laser irradiating method is as follows:
[0222] The glass substrate, as shown in FIG. 8, is mounted on a
convex stage, and the edges of the glass substrate are fixedly
pushed by appropriate pushers 803 made of metal or the like, to
thereby deform the substrate into a convex shape.
[0223] The stage has a mechanism that allows heated helium gas to
flow out and circulate by a pump 904, under the substrate 901, as
shown in FIG. 9, to thereby keep the substrate at a desired
temperature.
[0224] In this situation, a laser processing is conducted on the
substrate. A laser beam is moved back and forth, right and left,
and is irradiated on the substrate so as to be overlapped with each
other. Paying an attention to a certain point on the substrate, the
number of times of laser irradiation is 2 to 5.
[0225] It should be noted that, because the substrate to be
irradiated is warped in the form of a convex, the glass substrate
is moved vertically with respect to the laser beam so that the
focal point of the laser beam is always put on the substrate. Since
the thickness of the substrate, the shape of the convex surface,
and so on are found in advance, the height of the substrate is
controlled on the basis of those data so that annealing can be
uniformly conducted on the convex substrate surface, keeping the
focal point constant.
[0226] The height of the substrate may be fixed, and a lens is so
adjust as to move the focal point in such a manner that the focal
point of the laser beam is always set on the substrate.
[0227] Also, a distance to a surface to be irradiated is measured
using a displacement gauge or the like, on the basis of which the
height of the substrate or the focal point may be automatically
changed. Moreover, the substrate temperature at the time of
irradiating a laser beam is 200.degree. C.
[0228] Thereafter, the pushers are detached from the substrate, and
the substrate is cooled, to thereby flatten the substrate with the
contraction of the silicon film (FIG. 1B).
[0229] In this manner, the silicon film uniform in crystallinity
within substrate surface and the flat substrate having the film can
be obtained.
[0230] TFT is fabricated in the same manner as that of the first
embodiment.
[0231] The distribution of the threshold voltage of the TFT is
extremely unified within the substrate surface in comparison with
the TFT manufactured without flattening the glass substrate, as in
the first embodiment.
Sixth Embodiment
[0232] A sixth embodiment will be described with reference to FIGS.
1A to 1F.
[0233] An under silicon oxide film 102 having a thickness of 2000
.ANG. is formed on a glass substrate 101 (In this embodiment, there
is used a Corning 1737 of 400.times.500 mm square and 0.7 mm
thickness. It is needless to say that another glass substrate such
as Corning 7059, OA2, NA45, etc., may be used.), and an amorphous
silicon film 103 having a thickness of 500 .ANG. is sequentially
formed on the under silicon oxide film 102 by plasma CVD.
[0234] Then, nickel acetate aqueous solution of 10 ppm is coated on
the surface of silicon, and a nickel acetate layer is formed by
spin coating. It is more preferable that a surface active agent is
added to the nickel acetate aqueous solution. Since the nickel
acetate layer is very thin, although it is not necessarily to be in
a film shape, it does not suffer from any problem in the subsequent
process (FIG. 1A).
[0235] Then, the glass substrate is thermally annealed at
550.degree. C. for four hours, to thereby crystallize the amorphous
silicon film. In this state, nickel serves as the core of crystal,
to thereby promote the crystallization of the silicon film. It
should be noted that the strain point of the Corning 1737 substrate
is 667.degree. C., and the annealing temperature of 550.degree. C.
is below the strain point.
[0236] After the thermal crystallization, when the glass substrate
is cooled, the silicon film contracts, to warp the glass substrate
in the form of a concave.
[0237] That the processing can be made at a low temperature (the
strain point of the Corning 1737 or less) for a short period of
time, that is, at 550.degree. C. for four hours is caused by the
function of nickel. The details are disclosed in Japanese Patent
Unexamined Publication No. 6-244104. The publication discloses that
the thermal annealing, for example, at 550.degree. C. (below the
strain point) for four hours is conducted so that the temperature
in thermally annealing does not exceed the strain point of the
glass substrate. However, the temperature is determined so that the
glass substrate is prevented from being remarkably deformed in
thermally crystallizing.
[0238] It is preferable that the concentration of a catalytic
element is 1.times.10.sup.15 to 1.times.10.sup.19 atoms/cm.sup.3.
When it is a high concentration equal to or more than
1.times.10.sup.19 atoms/cm.sup.3, a metallic nature is exhibited on
silicon, whereby the semiconductor characteristic disappears. The
concentration of the catalytic element in the silicon film in this
embodiment is 1.times.10.sup.17 to 5.times.10.sup.18 atoms/cm.sup.3
at the minimum in the film. It should be noted that those values
are the minimum values of the concentration of the catalytic
elements in the silicon film which has been analyzed and measured
by SIMS.
[0239] In order to further enhance the crystallinity of the
crystalline silicon film thus obtained, an excimer laser beam which
is a high-power pulse laser is irradiated onto the film while
heating the film. In this situation, the glass substrate which has
been warped in the form of a concave is flattened
simultaneously.
[0240] The laser irradiating method is as follows The laser
annealing unit shown in FIG. 12 is used as in the first
embodiment.
[0241] An oscillator as used is a model 3000-308 made by Lambda
Physic Corporation. The laser beam oscillated from the oscillator
is an XeCl excimer laser beam (308 nm in wavelength and 26 ns in
pulse width).
[0242] Another excimer laser as well as another type laser can be
used. It should be noted that a laser beam of a pulse oscillator
need to be used. The oscillated laser beam is introduced into an
optical system as shown in FIGS. 14A and 14B in order to deform the
beam shape.
[0243] The laser beam immediately before being shone onto the
optical system is rectangular to the degree of 3.times.2 cm.sup.2,
but it is processed by the optical system into a slender beam
(linear beam) of about 10 to 30 cm in length and 0.01 to 0.3 cm in
width.
[0244] The distribution of the energy density of the linear laser
beam that has passed through the optical system in the cross
direction is trapezoidal as shown in FIG. 15B. The energy of the
laser beam that has passed through the optical system is 1000
mJ/shot at the maximum.
[0245] The reason why the laser beam is processed into such a
slender beam is to improve the processability. When the linear beam
is irradiated onto an sample, if the length of the laser beam is
longer than the width of the sample, then the sample is moved in
one direction, thereby irradiating a laser beam onto the entire
sample.
[0246] Even though the length of the beam is shorter than the width
of the sample, troublesomeness in processing is saved in comparison
with the rectangular beam. However, in this case, there occurs the
necessity of moving the beam vertically and horizontally relatively
with respect to the sample.
[0247] The stage (base) for the substrate (sample) on which a laser
beam is irradiated is controlled by a computer and so designed as
to be moved perpendicularly to the line direction of the linear
laser beam. Also, it is so designed that the height of the
substrate can fluctuate.
[0248] If the stage is provided with a function for moving in the
line direction of the laser beam, even though the beam width is
shorter than the sample, a laser processing on the entire sample is
enabled.
[0249] The optical path in the interior of the optical system that
processes a laser beam into a linear laser beam will be
described.
[0250] The laser beam oscillated from a laser light source a and
incident to the optical system first passes through fly eye lenses
b and c.
[0251] The laser beam passes through a cylindrical convex lens d as
a first cylindrical lens, and a cylindrical convex lens e as a
second cylindrical lens provided for improving the uniformity of
the linear beam in the line direction, and is then converged by the
cylindrical convex lens g via a mirror f before being irradiated
onto the sample.
[0252] A distance of from the laser light source to the mirror g is
2000 mm, and a distance of from the mirror f to the surface to be
irradiated is 440 mm. The cylindrical lens g as used has a focal
distance of 100 mm.
[0253] The shape of the energy distribution of the laser beam at
the focal point is made trapezoidal by changing the lens g
vertically (i-direction).
[0254] The surface to be irradiated is moved vertically
(i-direction) relatively with respect to the lens g, thereby
deforming the shape of the energy distribution of the laser beam on
the surface to be irradiated (focal point) with a range of from a
nearly square shape to a nearly trapezoidal shape.
[0255] The optical system is not particularly limited if it is so
designed as to deform the laser beam to the shape required by the
present invention.
[0256] The optical system is not limited to that shown in FIGS.
14.A and 14B, but it may be provided with lenses B and C as shown
in FIGS. 13A and 13B.
[0257] The laser beam is shaped into a linear form, and the area of
a laser beam on the surface to be irradiated is 150 mm.times.0.4
mm. (The width of the linear laser beam is half of the maximum
energy value of the laser beam.)
[0258] The energy profile (energy distribution) of the linear laser
beam in the cross direction has an artificial trapezoidal
distribution of L1=0.1 mm and L2, L3=0.08 mm as shown in FIG. 15B,
which satisfies the inequality of 0.5L1.ltoreq.L2.ltoreq.L1,
0.5L.ltoreq.L3.ltoreq.L1. In this case, the focal depth can be
about .+-.400 .mu.m.
[0259] The degree of widening of the bottom of the trapezoidal
distribution is changed in accordance with a distance between the
final lens of the laser optical system and the surface to be
irradiated. A distance between the final lens of the laser optical
system and the surface to be irradiated is changed by the roughness
of the object to be irradiated during the laser processing.
[0260] With any change of the distance between the final lens and
the surface to be irradiated, the degree of widening of the bottom
of the trapezoidal distribution of the laser beam is changed. If a
range of the change is within a range of the above inequality, then
the focal depth of about .+-.400 .mu.m is obtained, and therefore
when the roughness of the surface to be irradiated is .+-.400 .mu.m
or less, the uniform laser processing is enabled.
[0261] The general laser beam having a square energy distribution
is about .+-.200 .mu.m or less in focal depth.
[0262] The glass substrate, as shown in FIG. 10, is mounted on a
U-shaped convex stage, and the edges of the glass substrate are
fixedly pushed by appropriate pushers made of metal or the like, to
curve the substrate into a U-shape.
[0263] The stage has a mechanism that allows heated helium gas to
flow out and circulate, under the substrate, as shown in FIG. 9, to
thereby keep the substrate at a desired temperature. The laser
processing is conducted while the linear laser beam is being
shifted relatively with respect to the object to be irradiated. A
direction in which the linear laser beam is shifted is
substantially perpendicular to the linear laser beam, and a
straight line contained in the U-shaped curved surface of the
substrate to be irradiated is substantially in parallel with the
linear laser beam.
[0264] Because the substrate to be irradiated is warped in the form
of a U-shape, as shown in FIG. 11, the glass substrate is moved
vertically with respect to the laser beam so that the focal point
of the laser beam is always put on the substrate, during the
irradiation of a laser beam.
[0265] Since the thickness of the substrate, the shape of the
convex surface, and so on are found in advance, the height of the
substrate is controlled on the basis of those data so that
annealing can be uniformly conducted on the U-shaped surface of the
substrate, keeping the focal point constant.
[0266] The height of the substrate may be fixed, and a lens is so
adjust as to move the focal point in such a manner that the focal
point of the laser beam is always set on the substrate.
[0267] Also, a distance to a surface to be irradiated is measured
using a displacement gauge or the like, on the basis of which the
height of the substrate or the focal point may be automatically
changed.
[0268] The substrate temperature at the time of irradiating a laser
beam is 200.degree. C.
[0269] Since the energy distribution of a laser beam to be
irradiated is trapezoidal, and the focal depth is about .+-.400
.mu.m, if a difference in level between the central portion and the
edge portion of the U-shaped convex stage is about .+-.400 .mu.m or
less, the laser annealing can be uniformly conducted within the
substrate surface even though the stage and the focal point are not
varied at all.
[0270] The laser annealing can be extremely uniformly conducted if
the stage or the focal point is varied in accordance with a
difference in level of the surface to be irradiated, using a laser
beam having such a focal depth.
[0271] The glass substrate on the stage is moved perpendicular to
the line width direction at 2.5 mm/s.
[0272] The laser irradiation conditions are that the energy density
of the laser beam is 100 to 500 mJ/cm.sup.2, in this example, 400
mJ/cm.sup.2, and the number of pulses is 200 pulses/s. It should be
noted that the energy density means the density of the upper bottom
portion (a portion having a maximum value) of the trapezoidal laser
beam.
[0273] The irradiation of the laser beam is conducted under the
above conditions. Paying an attention to a certain point of the
sample, the laser beams are irradiated at 32 steps. This is
because, since it takes 0.4 sec to allow one laser beam to pass, 32
pulses are irradiated on one location by irradiating one laser beam
in the scanning manner. In this example, in the above 32
irradiations, the initial several irradiations have the irradiated
energy density gradually increased, and the final several
irradiations have the irradiated energy density gradually
decreased.
[0274] This state is schematically shown in FIG. 16. In the first
half of 32 steps, the laser energy gradually increases (see A in
FIG. 16) whereas, in the latter half thereof, it gradually
decreases (see B in FIG. 16).
[0275] Also, in this example, the atmospheric control is not
particularly conducted, and the irradiation of the laser beam is
conducted in the atmosphere. It may be conducted under the vacuum
or the atmosphere of an inactive gas such as argon or helium,
hydrogen or nitrogen.
[0276] Thereafter, the pushers are detached from the substrate and
then cooled so that the substrate is flattened with the contraction
of the silicon film (FIG. 1B).
[0277] The silicon film uniform in crystallinity within the
substrate surface and the flat substrate having the film can be
obtained. Thereafter, a TFT is fabricated in the same manner as in
the first embodiment.
[0278] The distribution of the threshold voltage of the TFT thus
obtained is extremely unified within the substrate surface in
comparison with the TFT manufactured without flattening the glass
substrate, as in the first embodiment.
Seventh Embodiment
[0279] A seventh embodiment will be described with reference to
FIGS. 1A to 1F.
[0280] An under silicon oxide film 102 having a thickness of 2000
.ANG. is formed on a glass substrate 101 (In this embodiment, there
is used a Corning 1737 of 400.times.500 mm square and 0.7 mm
thickness. It is needless to say that another glass substrate such
as Corning 7059, OA2, NA45, etc., may be used.), and an amorphous
silicon film 103 having a thickness of 500 .ANG. is sequentially
formed on the under silicon oxide film 102 by plasma CVD.
[0281] In order to crystallize the amorphous silicon film, an
excimer laser beam which is a high-power pulse laser is irradiated
onto the film while heating the film.
[0282] A KrF excimer laser (248 nm in wavelength and 30 nsec in
pulse width) is used. The size of a laser beam is 30.times.20
mm.sup.2. The laser beam is irradiated with the energy density of
the laser beam being within 100 mJ/cm.sup.2 to 500 mJ/cm.sup.2, for
example, 370 mJ/cm.sup.2. The crystallinity is further enhanced by
irradiating a laser beam onto the film with an energy of about 220
mJ/cm.sup.2 in advance before the irradiation of the laser
beam.
[0283] In this state, in order to prevent the substrate from being
warped by the contraction of the silicon film after the silicon
film is crystallized and cooled, the laser irradiating method is
conducted as follows:
[0284] The glass substrate, as shown in FIG. 8, is mounted on a
convex stage, and the edges of the glass substrate are fixedly
pushed by appropriate pushers made of metal or the like, to thereby
deform the substrate into a convex shape.
[0285] The stage has a mechanism that allows heated helium gas to
flow out and circulate under the substrate, as shown in FIG. 9, to
thereby keep the substrate at a desired temperature.
[0286] In this situation, a laser processing is conducted on the
substrate. A laser beam is moved back and forth, right and left,
and is irradiated on the substrate so as to be overlapped with each
other.
[0287] Paying an attention to a certain point on the substrate, the
number of times of laser irradiation is 2 to 5.
[0288] Because the substrate to be irradiated is warped in the form
of a convex, the glass substrate is moved vertically with respect
to the laser beam so that the focal point of the laser beam is
always put on the substrate. Since the thickness of the substrate,
the shape of the convex surface, and so on are found in advance,
the height of the substrate is controlled on the basis of those
data so that annealing can be uniformly conducted on the convex
substrate surface, keeping the focal point constant.
[0289] The height of the substrate is fixed, and a lens is so
adjust as to move the focal point in such a manner that the focal
point of the laser beam is always set on the substrate.
[0290] Also, a distance to a surface to be irradiated is measured
using a displacement gauge or the like, on the basis of which the
height of the substrate or the focal point may be automatically
changed.
[0291] The substrate temperature at the time of irradiating a laser
beam is 200.degree. C.
[0292] Thereafter, the pushers are detached from the substrate, and
the substrate is cooled, to thereby flatten the substrate with the
contraction of the silicon film (FIG. 1B).
[0293] In this manner, the silicon film uniform in crystallinity
within substrate surface and the flat substrate having the film can
be obtained. Thereafter, a TFT is fabricated in the same manner as
in the first embodiment.
[0294] The distribution of the threshold voltage of the TFT thus
obtained is extremely unified within the substrate surface in
comparison with the TFT manufactured without flattening the glass
substrate, as in the first embodiment.
Eighth Embodiment
[0295] An eighth embodiment will be described with reference to
FIGS. 1A to 1F.
[0296] An under silicon oxide film 102 having a thickness of 2000
.ANG. is formed on a glass substrate 101 (In this embodiment, there
is used a Corning 1737 of 400.times.500 mm square and 0.7 mm
thickness. It is needless to say that another glass substrate such
as Corning 7059, OA2, NA45, etc., may be used.), and an amorphous
silicon film 103 having a thickness of 500 .ANG. is sequentially
formed on the under silicon oxide film 102 by plasma CVD.
[0297] In order to crystallize the amorphous silicon film, an
excimer laser beam which is a high-power pulse laser is irradiated
onto the film while heating the film. In this process, the glass
substrate which has been warped in the form of a concave is
flattened simultaneously.
[0298] In this embodiment, crystallization is conducted using a
laser annealing unit having the optical system shown in FIGS. 14A
and 14B, as in the fourth embodiment. Various conditions for the
laser annealing are the same as those in the fourth embodiment.
[0299] The glass substrate is mounted on a U-shaped convex stage as
shown in FIG. 10, which is disposed in a laser annealing chamber of
the laser annealing unit shown in FIG. 12, and the edges of the
glass substrate are fixedly pushed by pushers made of metal or the
like, to curve the substrate into a U-shape.
[0300] The stage has a mechanism that allows heated helium gas to
flow out and circulate under the substrate, as shown in FIG. 9, to
maintain the substrate at a desired temperature.
[0301] The laser processing is conducted while the linear laser
beam is being shifted relatively with respect to the object to be
irradiated. A direction in which the linear laser beam is shifted
is substantially perpendicular to the linear laser beam, and a
straight line contained in the U-shaped curved surface of the
substrate to be irradiated is substantially in parallel with the
linear laser beam.
[0302] Because the substrate to be irradiated is warped in the form
of a convex U-shape, as shown in FIG. 11, the glass substrate is
moved vertically with respect to the laser beam so that the focal
point of the laser beam is always put on the substrate, during the
irradiation of a laser beam.
[0303] Since the thickness of the substrate, the shape of the
convex surface, and so on are found in advance, the height of the
substrate is controlled on the basis of those data so that
annealing can be uniformly conducted on the U-shaped surface of the
substrate, keeping the focal point constant.
[0304] The height of the substrate may be fixed, and a lens is so
adjust as to move the focal point in such a manner that the focal
point of the laser beam is always set on the substrate.
[0305] Also, a distance to a surface to be irradiated is measured
using a displacement gauge or the like, on the basis of which the
height of the substrate or the focal point may be automatically
changed.
[0306] Since the energy distribution of a laser beam to be
irradiated is trapezoidal, and the focal depth is about .+-.400
.mu.m, if a difference in level between the central portion and the
edge portion of the U-shaped convex stage is about .+-.400 .mu.m or
less, the laser annealing can be uniformly conducted within the
substrate surface even though the stage and the focal point are not
varied at all.
[0307] That the laser annealing can be extremely uniformly
conducted if the stage or the focal point is varied in accordance
with a difference in level of the surface to be irradiated, using a
laser beam having such a focal depth.
[0308] Moreover, the substrate temperature at the time of
irradiating a laser beam is 200.degree. C.
[0309] Thereafter, the pushers are detached from the substrate, and
the substrate is cooled, to flatten the substrate with the
contraction of the silicon film (FIG. 1B).
[0310] In this manner, the silicon film uniform in crystallinity
within substrate surface and the flat substrate having the film can
be obtained. Thereafter, a TFT is fabricated in the same manner as
in the first embodiment.
[0311] The distribution of the threshold voltage of the TFT is
extremely unified within the substrate surface in comparison with
the TFT produced without flattening the glass substrate, as in the
first embodiment.
Ninth Embodiment
[0312] A process of producing a TFT in accordance with a ninth
embodiment will be described with reference to FIGS. 1A to 1F.
[0313] An under silicon oxide film 102 having a thickness of 2000
.ANG. is formed on a glass substrate 101 (In this embodiment, there
is used a Corning 7059 of 100 mm square. Another glass substrate
such as Corning 1737, OA2, NA45, etc., may be used.), and an
amorphous silicon film 103 having a thickness of 500 .ANG. is then
formed on the under silicon oxide film 102 by plasma CVD.
[0314] Then, nickel acetate aqueous solution of 10 ppm is coated on
the surface of silicon, and a nickel acetate layer is formed by
spin coating. It is more preferable that a surface active agent is
added to the nickel acetate aqueous solution. Since the nickel
acetate layer is very thin, although it is not necessarily to be in
a film shape, it does not suffer from any problem in the subsequent
process (FIG. 1A).
[0315] Then, the glass substrate is thermally annealed at
550.degree. C. for four hours, to crystallize the amorphous silicon
film. In this state, nickel serves as the core of crystal, to
promote the crystallization of the silicon film.
[0316] That the processing can be made at a low temperature (the
strain point of the Corning 7059 or less) for a short period of
time, that is, at 550.degree. C. for four hours is caused by the
function of nickel. The details are disclosed in Japanese Patent
Unexamined Publication No. 6-244104.
[0317] It is preferable that the concentration of a catalytic
element is 1.times.10.sup.15 to 1.times.10.sup.19 atoms/cm.sup.3.
When it is a high concentration equal to or more than
1.times.10.sup.19 atoms/cm.sup.3, a metallic nature is exhibited on
silicon, whereby the semiconductor characteristic disappears. The
concentration of the catalytic element in the silicon film in this
embodiment is 1.times.10.sup.17 to 5.times.10.sup.18 atoms/cm.sup.3
at the minimum in the film. It should be noted that those values
are the minimum values of the concentration of the catalytic
elements in the silicon film which has been analyzed and measured
by SIMS.
[0318] In this way, a crystalline silicon film is obtained.
[0319] In this situation, the glass substrate is warped on the
surface on which the crystalline silicon film is formed, so as to
be concave. A difference in level between the central portion and
the peripheral portion of the substrate is about 50 .mu.m. The
degree of warp is different depending on the size, the thickness
and the kind of the glass substrate.
[0320] In order to further enhance the crystallinity of the
crystalline silicon film, an excimer laser beam which is a
high-power pulse laser is irradiated onto the film.
[0321] In this embodiment, there is used the laser annealing unit
described with reference to the second embodiment shown in FIGS.
12, 13A and 13B.
[0322] FIG. 17 shows an example of the structure of the stage.
[0323] As an example of means for fixedly mounting the flattened
glass substrate on the stage, there is, for example, provided a
plurality of suction inlets 202 on the upper surface of a stage
201. The suction inlets 202 are holes, and a flat surface is formed
at portions where no suction inlet 202 exists.
[0324] FIG. 17B shows that a groove 212 is defined in the upper
surface of the stage 211. The groove 212 communicates with a
suction inlet 213 in the center of the stage, and a flat surface is
formed at portions where no groove 212 exists.
[0325] FIG. 17C shows that a plurality of protrusions 222 are
provided on the upper surface of a stage 221, and a flat surface is
formed by the upper surface of those protrusions 222 and the
peripheral portion of the stage. Also, a suction inlet 223 is
provided for making vacuous.
[0326] FIGS. 17A to 17D show that the glass substrate is
vacuum-sucked onto the flat surface by making vacuous through the
suction inlet 1223. In this manner, the lower surface of the glass
substrate is in close contact with the flat surface of the stage.
Then, in this state, the laser annealing is conducted.
[0327] Because the flat surface on the upper surface of the stage
is flat except for the portion taking part in the suction, the
glass substrate in contact with the above flat upper surface is
flattened in accordance with the flat surface of the stage.
[0328] In this vacuum-suction method, the location and the
detachment of the glass substrate are performed extremely readily
and for a short time. Also, because no obstruction that prevents
the irradiation of a laser beam exists on the glass substrate
surface, the laser beam is uniformly irradiated onto the entire
surface of the glass substrate.
[0329] The flat surface of the stage is preferably as flat as
possible. However, it is sufficient that the crystalline silicon
film on the glass substrate mounted on the flat surface can be
annealed using a linear laser beam so that the film has the
uniformity of a required level.
[0330] For example, the flat surface is so formed that a difference
in level on the surface to be irradiated, of the glass substrate is
at least the focal depth of the laser beam or less.
[0331] The method of bringing the glass substrate in close contact
with the stage is not limited to the above suction method, and any
kind of method may be used if the glass substrate can be flattened
and the laser annealing can be conducted.
[0332] As another method, for example, in FIG. 17D, the peripheral
portion or the edge portions on the upper surface of the glass
substrate 1101 are mechanically pushed and pressed (pressurized)
against the flat surface of the stage 1231 by pushers 1232, and in
this situation, the laser annealing may be conducted.
[0333] In that case, because the glass substrate can be flattened
by a stronger force than the vacuum suction, the glass substrate
which is strongly warped to the degree that it cannot be
sufficiently flattened by vacuum suction can be readily
flattened.
[0334] The method shown in FIG. 17D and the above suction method
may be used together.
[0335] The material of the stage is preferably quartz, metal,
ceramics or the like because they are high in heat resistance and
keep the flatness high. In this example, there is used the stage
having the structure shown in FIG. 17A.
[0336] The suction inlets 1202 shown in FIG. 17A are about 1 mm in
diameter and provided at the intervals of 10 mm.
[0337] The glass substrate 1101 is placed on the flat surface of
the stage 1201 in such a manner that a surface of the glass
substrate 1101 on which a crystalline silicon film 1103 has been
formed is directed upwardly, and vacuum is made from the suction
inlets 1202 so that the glass substrate 1101 is brought in close
contact with the stage.
[0338] The glass substrate 1101 is also flattened to the same
degree as a difference in level within the flat surface of the
stage 1201 in accordance with the flat surface of the stage
1201.
[0339] In addition to the structure of FIG. 17A, not only the glass
substrate 1101 is merely placed on the stage 1201, but also, after
the former is placed on the latter, vacuum is made in a state where
a press is applied to the upper surface of the substrate, in
particular, the upper surface of the peripheral portion, and vacuum
is then made so that the glass substrate is brought in close
contact with the stage. For example, pushers 1232 shown in FIG. 17D
is provided so that the upper surface of the peripheral portion of
the glass substrate 1101 is pushed, and vacuum is made to bring the
glass substrate in close contact with the stage. Then, the pushers
are detached from the glass substrate, and thereafter the laser
annealing is conducted.
[0340] FIGS. 18A to 18C show a process of irradiating a laser beam
in accordance with this embodiment.
[0341] The amorphous silicon film formed on the glass substrate
2101 is thermally crystallized to obtain a crystalline silicon film
2103, and after cooling, the glass substrate 2101 is warped. As
shown in FIG. 18A, the glass substrate 2101 is located on the stage
2201.
[0342] In FIG. 18B, the warped glass substrate 2101 is forcedly
corrected by flattening and mounting the glass substrate formed on
the stage 2201, in this example, by the suction inlet 2202. The
glass substrate mounted on the stage 2201 is flattened to the
degree of about 5 .mu.m in level.
[0343] In FIG. 18C, a linear laser beam is irradiated onto the
crystalline silicon film 2103 on the glass substrate which has been
flattened in the scanning manner.
[0344] In this way, with the glass substrate 2101 being flatly
mounted, the linear laser beam is uniformly irradiated onto the
crystalline silicon film which is a surface to be irradiated
without any shift of the focal points regardless of the glass
substrate per se being warped.
[0345] The irradiation of the laser beam is conducted with the
energy density of the laser beam being within 100 mJ/cm.sup.2 to
500 mJ/cm.sup.2, for example, 370 mJ/cm.sup.2. The crystallinity is
further enhanced by irradiating a laser beam onto the film with an
energy of about 220 mJ/cm.sup.2 in advance before the above
irradiation of the laser beam, as the two-step irradiation.
[0346] The irradiation of the laser beam is conducted while the
linear laser beam is being shifted relatively with respect to an
object to be irradiated, that is, a crystalline silicon film. A
direction in which the linear laser beam is shifted is
substantially perpendicular to the linear laser beam (FIG. 14B,
h-direction). In this situation, paying an attention to a certain
point on the substrate, the laser beam of 2 to 40 shots, for
example, 32 shots is irradiated on the substrate. Also, the
substrate temperature at the time of irradiating the laser beam is
200.degree. C. (FIG. 1B).
[0347] In this way, a crystalline silicon film is fabricated. The
crystalline silicon film thus fabricated becomes sufficiently
uniform because the dispersion of the mobility within the substrate
surface is about .+-.10%.
[0348] In the crystalline silicon film which has been annealed by a
laser beam not through the flattening process shown in this
embodiment, the dispersion of mobility within the substrate surface
is about .+-.15 to 40%. Thus, the sufficient uniformity. cannot be
obtained.
[0349] On the basis of the crystalline silicon film thus
fabricated, the TFTs of 400.times.300 pieces within the manufacture
area of 40.times.50 mm.sup.2 is fabricated in accordance with the
producing process as described in the first embodiment.
[0350] The threshold voltage of the TFT is extremely unified within
the substrate surface in comparison with the TFT manufactured
without flattening the glass substrate, as shown in FIG. 5.
Tenth Embodiment
[0351] In the second embodiment, there is shown an example in which
the arrangement of the optical system and the structure of the
stage are used, which are different from those in the first
embodiment.
[0352] As in the first embodiment, referring to FIGS. 1A to 1F, an
under silicon oxide film 102 having a thickness of 2000 .ANG. is
formed on a glass substrate 101 (In this embodiment, there is used
a Corning 1737 of 300.times.300 mm square and 0.7 mm thickness. It
is needless to say that another glass substrate such as Corning
7059, OA2, NA45, etc., may be used.), and an amorphous silicon film
103 having a thickness of 500 .ANG. is sequentially formed on the
under silicon oxide film 102 by plasma CVD.
[0353] Then, nickel acetate aqueous solution of 10 ppm is coated on
the surface of silicon, and a nickel acetate layer is formed by
spin coating. It is more preferable that a surface active agent is
added to the nickel acetate aqueous solution. Since the nickel
acetate layer is very thin, although it is not necessarily to be in
a film shape, it does not suffer from any problem in the subsequent
process (FIG. 1A).
[0354] Then, the glass substrate is thermally annealed at
550.degree. C. for four hours, to thereby crystallize the silicon
film 103. In this state, nickel serves as the core of crystal, to
promote the crystallization of the amorphous silicon film. The
strain point of the Corning 1737 substrate is 667.degree. C., and
the annealing temperature of 550.degree. C. is below the strain
point.
[0355] After the thermal crystallization, when the glass substrate
is cooled, the silicon film contracts so that the substrate
produces the concave warp.
[0356] That the processing can be made at a low temperature (the
strain point of the Corning 1737 or less) for a short period of
time, that is, at 550.degree. C. for four hours is caused by the
function of nickel. The details are disclosed in Japanese Patent
Unexamined Publication No. 6-244104. The publication discloses that
the thermal annealing, for example, at 550.degree. C. (below the
strain point) for four hours is conducted so that the temperature
in thermally annealing does not exceed the strain point of the
glass substrate. However, the temperature is determined so that the
glass substrate 101 is prevented from being remarkably deformed in
thermally crystallizing.
[0357] In this state, the glass substrate is curved along the
surface on which the crystalline silicon film is formed, into a
concave portion.
[0358] In this example, a difference in level between the central
portion and the peripheral portion of the glass substrate is about
200 .mu.m. The degree of the warp is different depending upon the
size, the thickness and the kind of the glass substrate. In order
to further enhance the crystallinity of the crystalline silicon
film, an excimer laser beam which is a high-power pulse laser is
irradiated onto the film.
[0359] The laser annealing unit has the structure shown in FIG. 6
as in the first embodiment.
[0360] An oscillator as used is an EX748 made by LUMNICS
Corporation. The laser beam oscillated from the oscillator is a KrF
excimer laser beam (248 nm in wavelength and 25 ns in pulse
width).
[0361] Another excimer laser as well as another type laser can be
used. It should be noted that a laser beam of a pulse oscillator
need to be used.
[0362] The oscillated laser beam is introduced into an optical
system as shown in FIGS. 14A and 14B in order to deform the beam
shape.
[0363] The laser beam immediately before being shone onto the
optical system is rectangular to the degree of 3.times.2 cm.sup.2,
but it is processed by the optical system into a slender beam
(linear beam) of about 10 to 30 cm in length and 0.01 to 0.3 cm in
width. The energy of the laser beam that has passed through the
optical system is 800 mJ/shot at the maximum.
[0364] The reason why the laser beam is processed into such a
slender beam is to improve the processability. When the linear beam
is irradiated onto an sample, if the length of the laser beam is
longer than the width of the sample, then the sample is moved in
one direction, thereby irradiating a laser beam onto the entire
sample.
[0365] Even though the length of the beam is shorter than the width
of the sample, troublesomeness in processing is saved in comparison
with the rectangular beam. However, in this case, there occurs the
necessity of moving the beam back and forth, right and left
relatively with respect to the sample.
[0366] The stage (base) for the substrate (sample) on which a laser
beam is irradiated is controlled by a computer and so designed as
to be moved perpendicularly (FIG. 8, I-direction) to the line
direction of the linear laser beam.
[0367] If the stage is provided with a function for moving in the
line direction of the laser beam, even though the beam width is
shorter than the sample, a laser processing on the entire sample is
enabled.
[0368] The optical system that processes a laser beam into a linear
laser beam may be the same as that in other embodiments.
[0369] The optical system is not limited if it can deform a laser
beam into the beam shape required by the present invention.
[0370] The laser beam is shaped into a linear form, and the area of
a laser beam on the surface to be irradiated is 300 mm.times.1 mm.
The width of the linear laser beam is half of the maximum energy
value of the laser beam.
[0371] The glass substrate warped in the form of a concave through
the thermally crystallizing process is forcedly flattened and fixed
by the stage (base) of the laser annealing unit.
[0372] In this example, the stage having the structure shown in
FIG. 17D is used. In FIG. 17D, the pusher 232 in this example is
made of ceramic, but it may be made of metal, quartz or the like.
It is desirable that it may be made of a material high in heat
resistance and hard in thermal expansion.
[0373] The pusher 232, when the glass substrate 101 is transferred
and mounted on the stage 231, is automatically pressed on the upper
peripheral portion of the glass substrate 101 in such a manner that
the glass substrate 101 is fixedly brought in close contact with
the stage.
[0374] The glass substrate 101 is flattened in accordance with the
flat surface of the stage 231 and fixed thereto. The flattened
glass substrate is about 10 .mu.m in a difference in level within
the surface.
[0375] In this manner, a laser beam is irradiated onto the glass
substrate located on the stage (base).
[0376] The irradiation of the laser beam is conducted while the
linear laser beam is being shifted relatively with respect to an
object to be irradiated, that is, a crystalline silicon film. A
direction in which the linear laser beam is shifted is
substantially perpendicular to the linear laser beam (FIG. 13B,
I-direction). In this situation, paying an attention to a certain
point on the substrate, the laser beam of 2 to 20 shots, for
example, 15 shots is irradiated on the substrate.
[0377] The irradiation of the laser beam is conducted with the
energy density of the laser beam being within 100 mJ/cm.sup.2 to
500 mJ/cm.sup.2, for example, 370 mJ/cm.sup.2. The crystallinity is
further enhanced by irradiating a laser beam onto the film with an
energy of about 220 mJ/cm.sup.2 in advance before the above
irradiation of the laser beam, as the two-step irradiation. Also,
the substrate temperature at the time of irradiating the laser beam
is 200.degree. C. (FIG. 1B).
[0378] Moreover, in this example, the atmospheric control is not
particularly conducted, and the irradiation of the laser beam is
conducted in the atmosphere. It may be conducted under the vacuum
or the atmosphere of an inactive gas such as argon or helium,
hydrogen or nitrogen.
[0379] In this way, the crystalline silicon film having the uniform
crystallinity within the substrate surface can be obtained.
[0380] Thereafter, a TFT is fabricated using the crystalline
silicon film as in the first embodiment.
[0381] The threshold voltage of the TFT is extremely unified within
the substrate surface in comparison with the TFT manufactured
without flattening the glass substrate.
[0382] In the present invention, the substrate on which the film
has been formed can be restrained from being warped after being
heated and cooled, whereby the substrate can be flattened.
[0383] In the present invention, the glass substrate on which the
crystalline silicon film is formed can be flattened, and the
crystalline silicon film having the uniform and high crystallinity
within the substrate surface can be obtained even after the laser
irradiation process.
[0384] The crystalline silicon TFT uniform in the threshold value
voltage within the substrate surface can be fabricated.
[0385] The present invention is effective particularly in the case
where the area of the glass substrate is large in producing a large
number of TFTs on the glass substrate.
[0386] In forming a liquid crystal display using the glass
substrate, the cell pair can be made readily and surely since the
substrate is flat.
[0387] As described above, the present invention is useful from the
industrial viewpoint.
* * * * *